‘One’ year on, Neptune still a mystery

Neptune has just completed its first full orbit since its discovery 165 years ago.

The eighth and most distant planet from the Sun was the first planet discovered after being predicted by scientists using mathematics. British astronomer Sir William Herschel and his sister Caroline found Uranus in 1781. Shortly afterwards Herschel noticed the orbit of Uranus did not match the predictions based on Newton’s theory of gravity.

In 1821, French astronomer Alexis Bouvard speculated another planet was tugging on Uranus, altering its orbital motion.

Astronomers Urbain Le Verrier of France and John Couch Adams of England independently predicted the mystery planet’s location 20 years later by measuring how the gravity of a hypothetical object would affected Uranus’s orbit.

Le Verrier sent his predictions of the mystery planet’s location to German astronomer Johann Gottfried Galle at the Berlin Observatory.

Galle found and identified Neptune as a planet on September 23, 1846.

But he was not the first to see Neptune.

In December 1612, Galileo recorded Neptune in his notebook, while observing Jupiter and its moons, but as a star.

The next month he noted the ‘star’ appeared to have moved relative to other stars.

But because Galileo failed to identify Neptune as a planet, he cannot be credited with its discovery.

Mystery remains

Macquarie University planetary scientist Dr Craig O’Neill says

we really have not learnt all that much more about Neptune since then.

“It’s location at the edge of the solar system makes it a bit of

a black hole from a knowledge point of view,” says Dr O’Neill.

“Neptune’s so far away, it takes 165 Earth years to complete one orbit around the Sun. From Neptune, the Sun looks like a point of light no brighter than Venus does from Earth.”

Dr O’Neill says most of what we know about Neptune comes

from Voyager 2’s 1989 fly-by and the Hubble Space Telescope.

“Neptune’s atmosphere is hydrogen, helium and methane,” he said.

“We think its blue colour comes from methane absorbing red light. But Uranus has a similar atmosphere, yet it’s cyan in colour.”

Scientists are also still speculating about Neptune’s supersonic winds, which are the fastest in the Solar System.